Analysis of Cylinder Pressure and Heat Release Rate With Exhaust Emission and Fuel Consumption In a Diesel Engine

Individual cylinder pressure based feedback control is an ideal method to optimize engine operation. In-cylinder pressure is a fundamental combustion variable, which can be used to characterize the combustion process for each combustion event. Optimal engine control can be maintained by monitoring the pressure in each cylinder and using this information for feedback control in order to minimize the exhaust gas emissions. The availability of low cost and robust pressure sensors, such as the glow plug installed pressure sensor (GPPS) is expected to lead to the wide adoption of cylinder-pressure-based engine control diesel engines. In order to identify the most effective parameters to use when controlling combustion based on cylinder pressure information, a series of experiments were carried out with varying injection timings and EGR levels. This paper presents the results obtained and discusses the potential key parameters that may be used for closed loop control using an in-cylinder pressure transducer. This paper will also look at the combustion properties for combined hydrogen/diesel combustion. It was necessary to find an appropriate running point for the engine in purely diesel mode to act as a base point. Once this point was found, an experimental test matrix for the combined EGR and hydrogen investigation was set up.

EXPERIMENTAL TEST MATRIX

The injection strategy for this part of the investigation was a single injection at 1200 bar, maintained via the high-pressure common rail, at injection timings varying from 14 CAD BTDC and 0 CAD BTDC. The EGR level was varied, in 5% steps, from 0% and 40%. A set operating point of 1500 rpm and 2.7 bar BMEP was used. The exhaust gas emissions were all measured. This information was then used to plot correlations between the in-cylinder pressure and the related combustion properties and the resultant exhaust gas emissions.

The results are obtained for a range of EGR and fuel injection timings.

The followings are a summary of results obtained. Figure 1 shows that with increasing levels of EGR the 50 % MFB point is delayed. It can also be seen that higher levels of EGR have more of an effect than lower levels as the lines diverge with increasing EGR. This effect becomes more pronounced as the injection timing is retarded.

From Figure 2 it can be seen that as the EGR level is increased, the maximum in-cylinder pressure decreases. This is a known effect of EGR addition. The maximum pressure decreases as the injection timing is retarded. As the injection timing

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From Figure 3 it can be seen that increasing the EGR level delays the position of the maximum in-cylinder pressure. The position of the maximum pressure is also delayed as the injection timing is retarded. As the injection timing is retarded, the effect of the EGR is increased.

From Figure 4 it can be seen that with increasing levels of EGR, the maximum pressure rise rate decreases. As the injection timing is retarded, the pressure rise rate decreases. The effects of EGR appear to be greater with retarded injection. The results of different injection timings are not well separated. Hence, this is not a particularly reliable parameter. Figure 5 shows that the position of the maximum pressure rise rate is delayed with increased levels of EGR. It can also be seen that as the levels of EGR increases, the delay is also increased. As the injection timing is retarded, the position of the maximum pressure rise rate is delayed. This is all due to the increased ignition delay.

Figure 6 shows that the motoring pressure at 20 crank angle degrees before TDC decreases as the level of EGR increases, as was the case for the maximum in-cylinder pressure. The motoring pressure decreases as the injection timing is retarded. This is a parameter which needs further investigation before it could be recommended for a closed loop control system.

From Figure 7 it can be seen that increasing EGR decreases the emission of nitrogen oxides (NOX).Retarding the injection timing also decreases the emission of NOX. At earlier injection timings, the effect of EGR is more pronounced, as can be seen from the converging lines in Figure 7.

Figure 8 clearly shows that retarding the injection timing increases the total unburnt hydrocarbons (THC). As is known, increasing EGR level also increases the total unburnt hydrocarbons.The combined effect of retarded injection and high levels of EGR means a significant increase in the totalunburnt hydrocarbons. This makes it necessary to optimise the injection strategy to minimise the THC emissions.

Although not quite so clear, Figure 9 shows that increasing the EGR by a small percentage does not have much of an effect on the smoke number. With more than approximately 20% EGR, increasing the EGR significantly increases the smoke number. Broadly, retarded injection timing increases smoke number after the 20% EGR level. In can be seen from Figure 10 that increasing EGR levels increases the emissions of carbon monoxide (CO).It can also be seen that retarding the injection timing increases the emission of carbon monoxide. The effect of EGR increases both with retarded injection timing and increasing EGR levels.

Figure 11 shows that the specific fuel consumption decreases with EGR. The specific fuel consumption also decreases with retarded injection timing. Figure 32 also shows that this parameter is variable, and not appropriate for use as a closed loop control.

In Figure 12 EGR increases towards the bottom of the graph Injection timing is retarded from left to right Figure 33 shows that the position of the 50% MFB is retarded as nitrogen oxides are decreased.

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Unlike in the previous figure, in Figure 13 EGR increases towards the top of the graph. The injection timing is still retarded from left to right. From Figure 13 it can be seen that as the 50% MFB position is delayed the total unburnt hydrocarbons increases. This is the usual diesel trade of off:as NOX is decreased, THC increases.

Similar to Figure 13, Figure 14 follows EGR increases towards the top of the graph. Injection timing is retarded from left to right.As was the case for the total unburnt hydrocarbons, as the 50% position is delayed, the filter smoke number is increased, as shown in Figure14.This directly follows the THC emissions, which contributes to the smoke number, and is a direct result of the higher incomplete combustion. As with the previous two figures, with Figure 15 EGR increases towards the top of the graph Injection timing is retarded from left to right. It follows that as the 50% MFB position is delayed, and therefore the ignition delay, there is less time for the combustion event to occur. This means that incomplete combustion was increased, resulting in higher carbonmonoxide emissions, as can be seen in Figure 15

For Figure 16 EGR increases towards the bottom of the graph Injection timing is retarded from left to right. As the position of the 50% MFB is delayed, the specific fuel consumption increases, as is shown in Figure 16. The increase in incomplete combustion means lower fuel efficiency, and therefore higher fuel consumption.

For Figure 17 EGR increases from the top to the bottom of the graph. Injection timing is retarded from left to right. As the position of the maximum rate of pressure rise rate is retarded, the NOX emissions decrease, as shown in Figure 17.This decrease in NOX emissions corresponds to the in-cylinder pressure decrease brought about by the increasing EGR levels, as well as the decreased

In order to find an appropriate base point for the hydrogen experiments, an investigation of injection pressures and timings for a single diesel injection strategy was conducted. Injection pressures were varied from 400 to 1400 bar. The injection timing was varied between 0 and 14 Crank Angle Degrees Before Top Dead Centre (CAD BTDC).

As can be seen from Figure 18, NOX emissions increase as the injection timing is advanced, and as the injection pressure is increased. This is to be expected, as NOX emissions increase with temperature and with lower flame speeds. Earlier injection timings allows for a higher portion of the diesel fuel to be burnt during the premixed phase of combustion. This leads to higher rates of in-cylinder pressure rise, higher maximum in-cylinder pressures, and therefore higher temperatures. Conversely, the total unburnt hydrocarbons decrease as the injection timing is retarded and there is more time to allow for complete combustion. As the injection complete combustion. This tradeoff is typical of diesel combustion.

In order to investigate the effects of combined hydrogen/diesel combustion with EGR addition, various levels of EGR and hydrogen addition were used. The EGR levels were varied between 0% and 40%, where the engine was capable of running this level, in 5% steps. Hydrogen was supplied as shown in the previous chapter, and the levels were varied between 0 and 10% of the volume of the inlet charge, in 2% vol. steps. In the case of running both hydrogen and EGR, the hydrogen always replaced air. Two engine speeds were run, 1500rpm and 2500rpm, and 3 engine loads. The 3 engine loads were 0 bar BMEP, 2.7 bar BMEP and 5.4 bar BMEP, referred to as no load, medium load and high load throughout this paper. Since not all combinations of the speeds and loads were capable of supporting the maximum levels of hydrogen and EGR, Table 1 shows the achieved levels for each operating point. Table 1 Experimental Test Matrix

As described in the previous chapter, the in-cylinder pressure data was captured using a pressure transducer connected to a PC running LabVIEW via a charge amplifier. This in-cylinder pressure data, combined with the crank angle data captured simultaneously from the shaft encoder, was used to analyse the combustion process. The data

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where γ=1.3 The heat release rate trace was used to determine the nature of the combustion. As can been seen by comparing Figure 19 and Figure20, the addition of hydrogen has increased the maximum heat release rate. This is due to the increased ignition delay, and the great proportion diesel fuel being burned in the premixed phase of combustion. This increased rate of heat release is only true while stable combustion can be maintained, as will be discussed later in this section. Looking at Figure 20 it can be seen that the maximum rate of heat release actually decreases slightly, although the hydrogen level supplied has increased. This shows that the limit of stable combustion has been reached, and the benefits of the hydrogen addition will begin to decrease.

As can be seen from Figure 21 the maximum pressure for medium and highloads increases with hydrogen at 2500rpm. For the case of no load, increasing low loads. Unstable combustion was taken to be a greater than 5% Coefficient of Variation (CoV) of maximum in-cylinder pressure. (This has also been observed in previous studies where at low loads, the in-cylinder conditions were not always favourable for the efficient oxidation of hydrogen and the engine combustion stability could deteriorate Tsolakis (2004)].) There is higher maximum cylinder pressure with no EGR, due to the effect of EGR of lowering the maximum pressure.

The 1500 rpm case is slightly different from the 2500 rpm case, as can be seen from Figure 48. The onset of unstable combustion at 1500 rpm occurs at a lower load than at 2500 rpm. The advantages of hydrogen induction are clearer at high loads at low speeds.

As can be seen from Figure 22 as the hydrogen addition level increases the ignition delay increases, except in the no load case. Ignition delay was taken as the time between the start of fuel injection signal and the start of combustion. The start of combustion in this instance was taken as the point on the heat release rate curve after injection where the value changed from negative to positive [Heywood (1988)]. The decreased ignition delay in the no load case is interesting as it is not what would normally be expected with hydrogen addition, but is also reflected in the unusual trends in the emissions. The no load case needs further investigation to establish the exact chemical kinetics at this point. It may also be useful to measure the hydrogen emissions to establish whether or not the hydrogen is burning as the combustion at this point is unstable.

Hydrogen addition also resulted in increase of the peak heat release rate until the onset of unstable combustion. This is a result of the longer ignition delay, and therefore a greater volume of diesel fuel being burnt during the premixed phase of combustion. This in turn makes the combustion noisier.

Similarly as shown in Figure 23, the maximum rate of cylinder pressure rise increases with hydrogen (until the combustion becomes unstable)for medium and high loads, but decreases in the case of no load. With EGR the maximum rate of pressure rise was decreased. The increased maximum rate of cylinder pressure rise means that the combustion using hydrogen is noisier than using exclusively diesel.

In the case of the 1500 rpm speed, there is an unexpected dip in the rate of maximum in-cylinder rise at 4% vol. hydrogen addition for medium and high loads.

This paper has looked at various combustion parameters and their correlations with the exhaust pressure transducer. The effects of hydrogen addition on the combustion parameters are also examined.

1. Lee, CS, Lee, KH and Kim, DS, 2003, Experimental and numerical study on the combustion characteristics of partially premixed charge compression ignition engine with dual fuel, Fuel, vol. 82, pp. 553-560. 2. Muraro W. Shiraiwa, N. M., Monte, M., Figueiredo F.A. De Almedia, A., Moura, T. M. and Sanchez, C. G., 2005, Evalution of Engine Running with Gas of Low Power Heat Rate from Biomass (Rice Husk) Produced by Gasifier, SAE paper 2005-01-2185 3. Rakopoulos, C.D., Scott, M.A., Kyritsis, D.C., and Giakoumis, E.G., 2008, Availability analysis of hydrogen/natural gas blends combustion in internal combustion engines, Energy, vol. 33, pp. 248-255. 4. Tsolakis, A and Megaritis, A,2004,Exhaust Gas Fuel Reforming for Diesel Engines – A Way to Reduce Smoke and NOx Emissions Simultaneously, SAE paper No. 2004-01-1844. 5. Tsolakis,A., Megaritis, A.,Yap,D.and Abu-Jrai, A., 2005c, Combustion characteristics and exhaust gas emissions of a diesel engine supplied with reformed EGR, SAE paper No. 2005-01-2087. 6. Twigg, M.V., 2006, Progress and future challenges in controlling automotive exhaust gas emissions, Applied Catalysis B Environmental 70:2-15